(459c) Chabazite SAPO-34 Zeolite Membranes for Krypton/Xenon Separation: Enhanced Separation Performance and Process Modeling

Nuclear power is considered to be an economical means of carbon-free energy, but it requires effective ways to deal with radioactive waste products. Separation of radioactive ⁸⁵Kr from a Kr/Xe mixtures is one of the off-gas processing steps in the used nuclear fuel (UNF) recycling scheme proposed by the US DOE in reducing the volume of radioactive waste for storage. Instead of conventional cryogenic distillation, membrane-based separation could be an attractive separation method. Zeolite membranes offer an opportunity to exploit the size difference of Kr (kinetic diameter 0.36 nm) and Xe (0.396 nm) to achieve preferential permeation of the minority component (10 vol % of ⁸⁵Kr in gaseous mixture) from the mixture, resulting in an efficient separation with very low energy requirements. SAPO-34 is a silicoaluminophosphate zeolite molecular sieve with chabazite framework topology with uniform micropores, intermediate in size (0.38 nm) between the kinetic diameters of Kr and Xe. Our previous work indicated the potential for engineering the separation properties of these membranes, but revealed the need to considerably improve the membrane performance and evaluate the expected performance of a multistage membrane process for ⁸⁵Kr recovery. In this study, our main goals were to obtain SAPO-34 membranes with enhanced performance via thickness reduction and ion exchange, and to conduct process modeling that is useful for future technoeconomic analysis of zeolite membrane systems for Kr/Xe separation.

In the first part of this work, we demonstrate considerable thickness reduction of SAPO-34 membranes. Porous α-Al₂O₃ supports were seeded using steam-assisted conversion (SAC) methods to make a continuous seed layer followed by a conventional hydrothermal reaction for the secondary-growth process. The synthesized membranes showed dense membrane layers with thicknesses as low as 2.4 µm. The reduction of membrane thickness led to a large increase in Kr permeance from the previously reported [1] 7.5 GPU to 26.3 GPU with ideal selectivity of 21 at 298 K. Cation-exchanged membranes after thickness reduction showed moderate permeance reduction for both Kr and Xe, but showed large increases (more than 50%) in selectivity. The effects of the type and valency of the exchanged cations is discussed. These membranes were also shown to have excellent Kr selectivity in mixed-gas permeation during ambient and sub-ambient (-18°C) operation. In order to estimate the size of the separation system, process modeling of multistage membrane process was conducted using experimentally obtained permeances and selectivities. The required Xe purity in the retentate of each membrane stage is fixed as 99.9% and the permeate from each stage is sent to the next stage as feed till a Kr purity of 90+% (suitable for waste storage) is reached. Among the cation-exchanged SAPO-34 membranes, K-SAPO-34 membranes showed the best potential which requires a small membrane area (~7 m²) as well as the fewest number (~3) of stages for handling a 1 L/min Kr/Xe feed mixture. Based upon these findings, we conclude that the development of a compact, low cost membrane system for Kr separation is quite feasible.

[1] Y. H. Kwon, C. Kiang, E. Benjamin, P. Crawford, S. Nair, and R. Bhave, "Krypton-xenon separation properties of SAPO-34 zeolite materials and membranes". AIChE J. 63, 2, 761 (2017).